DOCSLIB.ORG

Nucleosynthesis: the S-, R- and P- Processes

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Nucleosynthesis: the S-, R- and P- Processes 1 Nucleosynthesis of the Heavy Elements Three basic processes can be identified by which heavy nuclei can be built by the continuous addition of protons or neutrons: • p-process (proton) • s-process (slow neutron) • r-process (rapid neutron) Capture of protons on light nuclei tend to produce only proton-rich nu- clei. Capture of neutrons on light nuclei produce neutron-rich nuclei, but which nuclei are produced depends upon the rate at which neutrons are added. Slow capture produces nuclei near the valley of beta stability, while rapid capture (i.e., rapid compared to typical beta-decay timescales) ini- tially produces very neutron-rich radioactive nuclei that eventually beta- decay towards the valley of beta stability. Some nuclei can be built by more than one process, as illustrated in Fig. 1. Figure 1: A small portion of the chart of the nuclides, illus- trating isotopes built by the three basic processes. Only stable isotopes are shown, and letters indicate the processes that can contribute to these isotopes. 2 Neutron Capture The difference between the s-process and r-process nucleosynthesis is controlled by beta decay rates of nuclei. The s-process path lies in the valley of beta stability, but the r-process path is shifted by many units toward neutron-richness. Fig. 2 shows the beta-decay timescales of radioactive neutron-rich nuclei, the positions of stable nuclei, neutron and proton magic numbers and the r-process path. Figure 2: Chart of the nuclides illustrating beta decay timescales (color), stable isotopes (black squares), magic numbers (hori- zontal and vertical black lines), the limit of known nuclei (di- agonal jagged black line), and the r-process path (purple line). Overlaid are the relative abundances of r-process nuclei, to il- lustrate the relation between magic numbers and abundances. For small neutron velocities v, i.e., energies up to a tens of keV, neutron capture cross sections vary as σ / v−1. Thus the product σv is roughly 3 constant for thermal energies. Characteristic cross sections are about 100- 1000 millibarns (10−25 − 10−24 cm2). For temperatures around 30 keV, thermal velocities of neutrons are about 3 × 108 cm s−3. Then, the lifetime of a nucleus against neutron capture is 1 3 × 1016 − 3 × 1017 τn ∼ ∼ s: nnσv nn s-process timescales are around 104 yr, while r-process timescales are around microseconds, indicating neutron densities of 105 − 106 cm−3 and 1023 − 1024 cm−3, respectively. Neutron densities intermediate to these might be expected to yield a nucleosynthesis pattern intermediate to the s- and r- processes. However, comparison to solar-system abundances indicates that this has not occurred. Neutron capture cross sections are generally smoothly varying with A because a captured neutron has an excitation energy of about 8 MeV (µn ' µp ≈ −8 MeV) where the density of levels if very high. However, for light nuclei or magic number nuclei (N or Z 2 [2; 8; 20; 28; 50; 82; 126]) this is not the case and the neutron capture cross section are very small, comparatively, being from 1 to 10 mb. s-process A quantitative analysis can be performed with the basic assumption that, except for magic number nuclei, beta decay timescales are all very rapid compared to the neutron cature rate. The nuclear chain thus indicated is unique, because one proceeds to heavier nuclei as indicated in Fig. 2, stepping horizontally until an unstable nucleus is reached, then moving on a diagonal path increasing the proton number by 1 and decreasing the neutron number by 1. One can let NA represent the abundance of nucleus A with out reference to Z, and we can ignore abundances of radioactive nuclei. The differential equation for abundances is then _ NA = − < σv >A nn (t) NA (t) + < σv >A−1 nn (t) NA−1 (t) ; where < σv >A is thermally averaged for each nucleus A. Under the as- −1 sumption that σ / v , one can write < σv >A≈ vT σA where vT is the thermal velocity which is a constant. Defining a neutron exposure τ by dτ = vT nn (t) dt; 4 we find 0 NA = −σANA + σA−1NA−1: This equation can only be solved with a suitable boundary condition. Be- cause neutron capture cross sections on light nuclei are so small, we can effectively assume that NA(0) = 0 for A < 56 and NA(0) = N56(0) for A > 56. On the other hand, for A = 209 the chain cerminates because Bi209 is the most massive stable nucleus. Nuclei with mass 210 decay by α decay to mass 206. We will ignore this complication. 0 The above equation has the property that NA < 0 if NA > (σA−1/σA)NA−1 and vice-versa. Thus it is self-regulating: NA decreases if it is too large with respect to NA−1 and increases if it is too small. If the process operates long enough, it will come to equilibrium: NA0 = 0, in which case σANA ≈ σA−1NA−1: In-between the magic numbers, the cross sections are large compared to their differences, so locally this is a good approximation to the solution. Over a large range of A, however, this result is a poor approximation, because of the small cross sections associated with magic nuclei. A proper general soution is a little tricky, but can be analyzed by using the Laplace transform 1 ^ −st Nk (s) = Nk (t) e dt Z0 using k = A − 55. Using the fact that the Laplace transform of a derivative is 1 0 −st ^ Nk (t) e dt = sNk (s) − Nk (0) ; Z0 the transformed equations and boundary condition are: sN^1 (s) = − σ1N^1 (s) + N1 (0) ; ^ ^ ^ sNk (s) = − σkNk (s) + σk−1Nk−1 (s) : The solution of these equations, by substitution, is ^ N1 (0) σk Nk (s) = : σk s + σk Yk Here the notation Πk indicates successive multiplications of arguments from subscript k to 1. Since the product σN is expected to be smoothly varying, 5 we choose to use that product as the dependent variable. We define ^ ^ σkNk (s) −1 k (s) ≡ = (1 + s/σk) : N1 (0) Yk The solution is then the inverse Laplace transform: 1 i1 (τ) ≡ σN (τ) = ^ (s) esτ ds k k 2πi k Z−i1 k σ −σiτ k = e σi : σk − σi Xi=1 kY=6 i Figure 3: The distribution function k(τ) using σ = 100. Left: Shown as a function of k for τ = 0:5012. Right: Shown as a function of τ for k = 50. This solution encounters severe numerical difficulties at large atomic weights. The sum must be taken over about 150 terms, which differ greatly in order of magnitude but still must be included because of their near can- cellations. It is therefore necessary to find an approximate solution. To illustrate the behavior of the exact solution, consider the idealized case in which all the cross sections are equal, σk = σ. The solution in that case, starting from ^ − k (s) = (1 + s/σ) k; 6 is σ (τ) = (στ)k−1 e−στ ; k (k − 1)! which is a Poisson-like distribution in both k and τ with maxima at kmax = −1 στ + 1 and τmax = σ (k − 1). Thus, as the neutron exposure τ increases, the maximum in the distribution moves to larger values of k. The abun- dance (distribution width) at the maximum N = ∆−1 ' (2πστ)−1=2 kmax kmax decreases (increases) with k. The approximate solution begins with this result. If we can find numbers λk and mk such that s −mk s −1 1 + ' Π 1 + λ k σ k k for small s, we could write an approximate solution in the general case as (λ τ)mk−1 k −λkτ k (τ) ' λk e : Γ (mk) It can be shown that the constraint that the first three moments of k(τ) for the approximate case equal that of the general case, i.e., 1 k (τ) dτ = 1; Z0 1 k −1 τ k (τ) dτ = σi ; Z0 i X 2 1 k k 2 −1 −2 τ k (τ) dτ = σi + σi ; 0 0 1 Z Xi Xi @ A yields 2 k −1 k −1 i=1 σi σ m = ; λ = i=1 i : k k −2 k k −2 P i=1 σi Pi=1 σi This solution is valid to bPetter than 10% accuracyP, which is small compared to the numerical uncertainty in the experimental products σkNk. Each function k(τ) begins at zero, rises to a single maximum at some value τk;max and then falls exponentially to zero at large τ. Magic numbers 7 produce abrupt changes in the distributions. Often, results are expressed in terms of the number of neutrons captured per initial iron seed nucleus instead of τ, a relationship which depends upon the values of the cross sections in the capture chain. Figure 4: The distribution function A for different labels of total neutron irradiation, labelled by nc, the average number of neutrons captured per initial seed nucleus (Fe). Each s-process event results in a different abundance pattern because the exposure τ varies from event to event. An often-used approximation is that −τ/τ the probability of an event is equal to Ge o with G and τo parameters. With this approximation, the abundance patter produced overall will be 1 (λ τ)mA−1 λ τ mA σ N ' Gλ e−τ/τo A e−λAτ dτ = G A o : A A A Γ (m ) 1 + λ τ Z0 A A o That such a simple model accurately reproduces solar system s-process abundances is taken as confirmation of the general correctness of the ideas.
Load more

Recommended publications
  • Neutron Interactions and Dosimetry Outline Introduction Tissue
    Outline • Neutron dosimetry Neutron Interactions and – Thermal neutrons Dosimetry – Intermediate-energy neutrons – Fast neutrons Chapter 16 • Sources of neutrons • Mixed field dosimetry, paired dosimeters F.A. Attix, Introduction to Radiological • Rem meters Physics and Radiation Dosimetry Introduction Tissue composition • Consider neutron interactions with the majority tissue elements H, O, C, and N, and the resulting absorbed dose • Because of the short ranges of the secondary charged particles that are produced in such interactions, CPE is usually well approximated • Since no bremsstrahlung x-rays are generated, the • The ICRU composition for muscle has been assumed in absorbed dose can be assumed to be equal to the most cases for neutron-dose calculations, lumping the kerma at any point in neutron fields at least up to 1.1% of “other” minor elements together with oxygen to an energy E ~ 20 MeV make a simple four-element (H, O, C, N) composition Neutron kinetic energy Neutron kinetic energy • Neutron fields are divided into three • Thermal neutrons, by definition, have the most probable categories based on their kinetic energy: kinetic energy E=kT=0.025eV at T=20C – Thermal (E<0.5 eV) • Neutrons up to 0.5eV are considered “thermal” due to simplicity of experimental test after they emerge from – Intermediate-energy (0.5 eV<E<10 keV) moderator material – Fast (E>10 keV) • Cadmium ratio test: • Differ by their primary interactions in tissue – Gold foil can be activated through 197Au(n,)198Au interaction and resulting biological effects
    [Show full text]
  • Neutrinoless Double Beta Decay
    REPORT TO THE NUCLEAR SCIENCE ADVISORY COMMITTEE Neutrinoless Double Beta Decay NOVEMBER 18, 2015 NLDBD Report November 18, 2015 EXECUTIVE SUMMARY In March 2015, DOE and NSF charged NSAC Subcommittee on neutrinoless double beta decay (NLDBD) to provide additional guidance related to the development of next generation experimentation for this field. The new charge (Appendix A) requests a status report on the existing efforts in this subfield, along with an assessment of the necessary R&D required for each candidate technology before a future downselect. The Subcommittee membership was augmented to replace several members who were not able to continue in this phase (the present Subcommittee membership is attached as Appendix B). The Subcommittee solicited additional written input from the present worldwide collaborative efforts on double beta decay projects in order to collect the information necessary to address the new charge. An open meeting was held where these collaborations were invited to present material related to their current projects, conceptual designs for next generation experiments, and the critical R&D required before a potential down-select. We also heard presentations related to nuclear theory and the impact of future cosmological data on the subject of NLDBD. The Subcommittee presented its principal findings and comments in response to the March 2015 charge at the NSAC meeting in October 2015. The March 2015 charge requested the Subcommittee to: Assess the status of ongoing R&D for NLDBD candidate technology demonstrations for a possible future ton-scale NLDBD experiment. For each candidate technology demonstration, identify the major remaining R&D tasks needed ONLY to demonstrate downselect criteria, including the sensitivity goals, outlined in the NSAC report of May 2014.
    [Show full text]
  • RIB Production by Photofission in the Framework of the ALTO Project
    Available online at www.sciencedirect.com NIM B Beam Interactions with Materials & Atoms Nuclear Instruments and Methods in Physics Research B 266 (2008) 4092–4096 www.elsevier.com/locate/nimb RIB production by photofission in the framework of the ALTO project: First experimental measurements and Monte-Carlo simulations M. Cheikh Mhamed *, S. Essabaa, C. Lau, M. Lebois, B. Roussie`re, M. Ducourtieux, S. Franchoo, D. Guillemaud Mueller, F. Ibrahim, J.F. LeDu, J. Lesrel, A.C. Mueller, M. Raynaud, A. Said, D. Verney, S. Wurth Institut de Physique Nucle´aire, IN2P3-CNRS/Universite´ Paris-Sud, F-91406 Orsay Cedex, France Available online 11 June 2008 Abstract The ALTO facility (Acce´le´rateur Line´aire aupre`s du Tandem d’Orsay) has been built and is now under commissioning. The facility is intended for the production of low energy neutron-rich ion-beams by ISOL technique. This will open new perspectives in the study of nuclei very far from the valley of stability. Neutron-rich nuclei are produced by photofission in a thick uranium carbide target (UCx) using a 10 lA, 50 MeV electron beam. The target is the same as that already had been used on the previous deuteron based fission ISOL setup (PARRNE [F. Clapier et al., Phys. Rev. ST-AB (1998) 013501.]). The intended nominal fission rate is about 1011 fissions/s. We have studied the adequacy of a thick carbide uranium target to produce neutron-rich nuclei by photofission by means of Monte-Carlo simulations. We present the production rates in the target and after extraction and mass separation steps.
    [Show full text]
  • The R-Process Nucleosynthesis and Related Challenges
    EPJ Web of Conferences 165, 01025 (2017) DOI: 10.1051/epjconf/201716501025 NPA8 2017 The r-process nucleosynthesis and related challenges Stephane Goriely1,, Andreas Bauswein2, Hans-Thomas Janka3, Oliver Just4, and Else Pllumbi3 1Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium 2Heidelberger Institut fr¨ Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany 3Max-Planck-Institut für Astrophysik, Postfach 1317, 85741 Garching, Germany 4Astrophysical Big Bang Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan Abstract. The rapid neutron-capture process, or r-process, is known to be of fundamental importance for explaining the origin of approximately half of the A > 60 stable nuclei observed in nature. Recently, special attention has been paid to neutron star (NS) mergers following the confirmation by hydrodynamic simulations that a non-negligible amount of matter can be ejected and by nucleosynthesis calculations combined with the predicted astrophysical event rate that such a site can account for the majority of r-material in our Galaxy. We show here that the combined contribution of both the dynamical (prompt) ejecta expelled during binary NS or NS-black hole (BH) mergers and the neutrino and viscously driven outflows generated during the post-merger remnant evolution of relic BH-torus systems can lead to the production of r-process elements from mass number A > 90 up to actinides. The corresponding abundance distribution is found to reproduce the∼ solar distribution extremely well. It can also account for the elemental distributions observed in low-metallicity stars. However, major uncertainties still affect our under- standing of the composition of the ejected matter.
    [Show full text]
  • Chapter 3 the Fundamentals of Nuclear Physics Outline Natural
    Outline Chapter 3 The Fundamentals of Nuclear • Terms: activity, half life, average life • Nuclear disintegration schemes Physics • Parent-daughter relationships Radiation Dosimetry I • Activation of isotopes Text: H.E Johns and J.R. Cunningham, The physics of radiology, 4th ed. http://www.utoledo.edu/med/depts/radther Natural radioactivity Activity • Activity – number of disintegrations per unit time; • Particles inside a nucleus are in constant motion; directly proportional to the number of atoms can escape if acquire enough energy present • Most lighter atoms with Z<82 (lead) have at least N Average one stable isotope t / ta A N N0e lifetime • All atoms with Z > 82 are radioactive and t disintegrate until a stable isotope is formed ta= 1.44 th • Artificial radioactivity: nucleus can be made A N e0.693t / th A 2t / th unstable upon bombardment with neutrons, high 0 0 Half-life energy protons, etc. • Units: Bq = 1/s, Ci=3.7x 1010 Bq Activity Activity Emitted radiation 1 Example 1 Example 1A • A prostate implant has a half-life of 17 days. • A prostate implant has a half-life of 17 days. If the What percent of the dose is delivered in the first initial dose rate is 10cGy/h, what is the total dose day? N N delivered? t /th t 2 or e Dtotal D0tavg N0 N0 A. 0.5 A. 9 0.693t 0.693t B. 2 t /th 1/17 t 2 2 0.96 B. 29 D D e th dt D h e th C. 4 total 0 0 0.693 0.693t /th 0.6931/17 C.
    [Show full text]
  • Electron Capture in Stars
    Electron capture in stars K Langanke1;2, G Mart´ınez-Pinedo1;2;3 and R.G.T. Zegers4;5;6 1GSI Helmholtzzentrum f¨urSchwerionenforschung, D-64291 Darmstadt, Germany 2Institut f¨urKernphysik (Theoriezentrum), Department of Physics, Technische Universit¨atDarmstadt, D-64298 Darmstadt, Germany 3Helmholtz Forschungsakademie Hessen f¨urFAIR, GSI Helmholtzzentrum f¨ur Schwerionenforschung, D-64291 Darmstadt, Germany 4 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA 5 Joint Institute for Nuclear Astrophysics: Center for the Evolution of the Elements, Michigan State University, East Lansing, Michigan 48824, USA 6 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA E-mail: [email protected], [email protected], [email protected] Abstract. Electron captures on nuclei play an essential role for the dynamics of several astrophysical objects, including core-collapse and thermonuclear supernovae, the crust of accreting neutron stars in binary systems and the final core evolution of intermediate mass stars. In these astrophysical objects, the capture occurs at finite temperatures and at densities at which the electrons form a degenerate relativistic electron gas. The capture rates can be derived in perturbation theory where allowed nuclear transitions (Gamow-Teller transitions) dominate, except at the higher temperatures achieved in core-collapse supernovae where also forbidden transitions contribute significantly to the rates. There has been decisive progress in recent years in measuring Gamow-Teller (GT) strength distributions using novel experimental techniques based on charge-exchange reactions. These measurements provide not only data for the GT distributions of ground states for many relevant nuclei, but also serve as valuable constraints for nuclear models which are needed to derive the capture rates for the arXiv:2009.01750v1 [nucl-th] 3 Sep 2020 many nuclei, for which no data exist yet.
    [Show full text]
  • Concrete Analysis by Neutron-Capture Gamma Rays Using Californium 252
    CONCRETE ANALYSIS BY NEUTRON-CAPTURE GAMMA RAYS USING CALIFORNIUM 252 Dick Duffey, College of Engineering, University of Maryland; Peter F. Wiggins, Naval Systems Engineering Department, U.S. Naval Academy; Frank E. Senftle, U.S. Geological Survey; and A. A. El Kady, United Arab Republic Atomic Energy Establishment, Cairo The feasibility of analyzing concrete and cement by a measurement of the neutron-capture or prompt gamma rays was investigated; a 100-ug californium-252 source was used to supply the neutrons. A lithium drifted germanium crystal detected the capture gamma rays emitted. The capture gamma rays from cement, sand, and 3 coarse aggregates-quartzite gravel, limestone, and diabase--:were studied. Concrete blocks made from these materials were then tested. The capture gamma ray response of the calcium, silicon, and iron in the concrete blocks was in accord with the elements identified in the mix materials. The principal spectral lines used were the 6.42 MeV line of calcium, the 4.93 MeV line of silicon, and the doublet of iron at about 7 .64 MeV. The aluminum line at 7. 72 MeV was ob­ served in some cases but at a lower intensity with the limited electronic equipment available. This nuclear spectroscopic technique offers a possi­ ble method of determining the components of sizable concrete samples in a nondestructive, in situ manner. In addition, the neutron-capture gamma ray technique might find application in control of the concrete and cement processes and furnish needed information on production operations and inventories. • FROM THE point of view of the geochemical analyst, concrete may be considered as rock relocated and reformed at the convenience of the engineer.
    [Show full text]
  • Star Factories: Nuclear Fusion and the Creation of the Elements
    Star Factories: Nuclear Fusion and the Creation of the Elements Science In the River City workshop 3/22/11 Chris Taylor Department of Physics and Astronomy Sacramento State Introductions! Science Content Standards, Grades 9 - 12 Earth Sciences: Earth's Place in the Universe 1.e ''Students know the Sun is a typical star and is powered by nuclear reactions, primarily the fusion of hydrogen to form helium.'' 2.c '' Students know the evidence indicating that all elements with an atomic number greater than that of lithium have been formed by nuclear fusion in stars.'' Three topics tonight: 1) how do we know all the heavier elements are made in stars? (Big Bang theory) 2) How do stars make elements as heavy as or less heavy than iron? (Stellar nucleosynthesis) 3) How do stars make elements heavier than iron? (Supernovae) Big Bang Nucleosynthesis The Big Bang theory predicts that when the universe first formed, the only matter that existed was hydrogen, helium, and very tiny amounts of lithium. If this is true, then all other elements must have been created in stars. Astronomers use spectroscopy to examine the light emitted by distant stars to determine what kinds of atoms are in them. We've learned that most stars contain nearly every element in the periodic table. The spectrum of the Sun In order to measure measure what kinds of atoms were around in the earliest days of the Universe, we look for stars that were made out of fresh, primordial gas. The closest we can get to this is looking at dwarf galaxies, which show extremely low levels of elements heavier than helium.
    [Show full text]
  • Radioactive Decay
    North Berwick High School Department of Physics Higher Physics Unit 2 Particles and Waves Section 3 Fission and Fusion Section 3 Fission and Fusion Note Making Make a dictionary with the meanings of any new words. Einstein and nuclear energy 1. Write down Einstein’s famous equation along with units. 2. Explain the importance of this equation and its relevance to nuclear power. A basic model of the atom 1. Copy the components of the atom diagram and state the meanings of A and Z. 2. Copy the table on page 5 and state the difference between elements and isotopes. Radioactive decay 1. Explain what is meant by radioactive decay and copy the summary table for the three types of nuclear radiation. 2. Describe an alpha particle, including the reason for its short range and copy the panel showing Plutonium decay. 3. Describe a beta particle, including its range and copy the panel showing Tritium decay. 4. Describe a gamma ray, including its range. Fission: spontaneous decay and nuclear bombardment 1. Describe the differences between the two methods of decay and copy the equation on page 10. Nuclear fission and E = mc2 1. Explain what is meant by the terms ‘mass difference’ and ‘chain reaction’. 2. Copy the example showing the energy released during a fission reaction. 3. Briefly describe controlled fission in a nuclear reactor. Nuclear fusion: energy of the future? 1. Explain why nuclear fusion might be a preferred source of energy in the future. 2. Describe some of the difficulties associated with maintaining a controlled fusion reaction.
    [Show full text]
  • Heavy Element Nucleosynthesis
    Heavy Element Nucleosynthesis A summary of the nucleosynthesis of light elements is as follows 4He Hydrogen burning 3He Incomplete PP chain (H burning) 2H, Li, Be, B Non-thermal processes (spallation) 14N, 13C, 15N, 17O CNO processing 12C, 16O Helium burning 18O, 22Ne α captures on 14N (He burning) 20Ne, Na, Mg, Al, 28Si Partly from carbon burning Mg, Al, Si, P, S Partly from oxygen burning Ar, Ca, Ti, Cr, Fe, Ni Partly from silicon burning Isotopes heavier than iron (as well as some intermediate weight iso- topes) are made through neutron captures. Recall that the prob- ability for a non-resonant reaction contained two components: an exponential reflective of the quantum tunneling needed to overcome electrostatic repulsion, and an inverse energy dependence arising from the de Broglie wavelength of the particles. For neutron cap- tures, there is no electrostatic repulsion, and, in complex nuclei, virtually all particle encounters involve resonances. As a result, neutron capture cross-sections are large, and are very nearly inde- pendent of energy. To appreciate how heavy elements can be built up, we must first consider the lifetime of an isotope against neutron capture. If the cross-section for neutron capture is independent of energy, then the lifetime of the species will be ( )1=2 1 1 1 µn τn = ≈ = Nnhσvi NnhσivT Nnhσi 2kT For a typical neutron cross-section of hσi ∼ 10−25 cm2 and a tem- 8 9 perature of 5 × 10 K, τn ∼ 10 =Nn years. Next consider the stability of a neutron rich isotope. If the ratio of of neutrons to protons in an atomic nucleus becomes too large, the nucleus becomes unstable to beta-decay, and a neutron is changed into a proton via − (Z; A+1) −! (Z+1;A+1) + e +ν ¯e (27:1) The timescale for this decay is typically on the order of hours, or ∼ 10−3 years (with a factor of ∼ 103 scatter).
    [Show full text]
  • 2.3 Neutrino-Less Double Electron Capture - Potential Tool to Determine the Majorana Neutrino Mass by Z.Sujkowski, S Wycech
    DEPARTMENT OF NUCLEAR SPECTROSCOPY AND TECHNIQUE 39 The above conservatively large systematic hypothesis. TIle quoted uncertainties will be soon uncertainty reflects the fact that we did not finish reduced as our analysis progresses. evaluating the corrections fully in the current analysis We are simultaneously recording a large set of at the time of this writing, a situation that will soon radiative decay events for the processes t e'v y change. This result is to be compared with 1he and pi-+eN v y. The former will be used to extract previous most accurate measurement of McFarlane the ratio FA/Fv of the axial and vector form factors, a et al. (Phys. Rev. D 1984): quantity of great and longstanding interest to low BR = (1.026 ± 0.039)'1 I 0 energy effective QCD theory. Both processes are as well as with the Standard Model (SM) furthermore very sensitive to non- (V-A) admixtures in prediction (Particle Data Group - PDG 2000): the electroweak lagLangian, and thus can reveal BR = (I 038 - 1.041 )*1 0-s (90%C.L.) information on physics beyond the SM. We are currently analyzing these data and expect results soon. (1.005 - 1.008)* 1W') - excl. rad. corr. Tale 1 We see that even working result strongly confirms Current P1IBETA event sxpelilnentstatistics, compared with the the validity of the radiative corrections. Another world data set. interesting comparison is with the prediction based on Decay PIBETA World data set the most accurate evaluation of the CKM matrix n >60k 1.77k element V d based on the CVC hypothesis and ihce >60 1.77_ _ _ results
    [Show full text]
  • Sensitivity of Type Ia Supernovae to Electron Capture Rates E
    A&A 624, A139 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935095 & c ESO 2019 Astrophysics Sensitivity of Type Ia supernovae to electron capture rates E. Bravo E.T.S. Arquitectura del Vallès, Universitat Politècnica de Catalunya, Carrer Pere Serra 1-15, 08173 Sant Cugat del Vallès, Spain e-mail: [email protected] Received 22 January 2019 / Accepted 8 March 2019 ABSTRACT The thermonuclear explosion of massive white dwarfs is believed to explain at least a fraction of Type Ia supernovae (SNIa). After thermal runaway, electron captures on the ashes left behind by the burning front determine a loss of pressure, which impacts the dynamics of the explosion and the neutron excess of matter. Indeed, overproduction of neutron-rich species such as 54Cr has been deemed a problem of Chandrasekhar-mass models of SNIa for a long time. I present the results of a sensitivity study of SNIa models to the rates of weak interactions, which have been incorporated directly into the hydrodynamic explosion code. The weak rates have been scaled up or down by a factor ten, either globally for a common bibliographical source, or individually for selected isotopes. In line with previous works, the impact of weak rates uncertainties on sub-Chandrasekhar models of SNIa is almost negligible. The impact on the dynamics of Chandrasekhar-mass models and on the yield of 56Ni is also scarce. The strongest effect is found on the nucleosynthesis of neutron-rich nuclei, such as 48Ca, 54Cr, 58Fe, and 64Ni. The species with the highest influence on nucleosynthesis do not coincide with the isotopes that contribute most to the neutronization of matter.
    [Show full text]